This invention relates to a microwave oven automatically performing a range heating , and more particularly to control of a magnetron and a heater employed in such a microwave oven.
In conventional microwave ovens, a humidity sensor is provided for sensing moisture or water vapor emanating from food being range cooked or a gas sensor is provided for sensing gas components such as alcohol. A microcomputer operates to control the operation of a magnetron and particularly, the time when the heating operation is completed, based on the results of the sensing of the humidity sensor or the gas sensor.
Japanese Laid-open Patent Application No. 61-269890 discloses an arrangement that sound waves emanating from the food boiling are sensed by a microphone and the sensing result is utilized for controlling the magnetron and this arrangement has recently been practiced.
In the arrangement in which the humidity sensor is employed, oil or melted fat contained in smoke given out from the food being cooked adheres to the surface of the humidity sensor, which lowers the sensitivity of the humidity sensor. Thus, the humidity sensor has a disadvantage in the operational reliability. In order to overcome this disadvantage, the humidity sensor is periodically heated by a heater so that stains are periodically removed from the humidity sensor surface. This improvement, however, increases the complexity of the circuit arrangement and the production cost.
In the arrangement employing the gas sensor, the temperature of the gas sensor needs to be usually maintained at about 300.degree. C., which also brings about the complexity of the circuit arrangement and high production cost.
On the other hand, in the case of sensing the sound waves produced from the food being cooked by the microphone, the sensing result is influenced by the motor vibration, external noise and the like. Consequently, the operational reliability is also in the low level.